Apparatus and method for adaptive amplitude equalization

Abstract

Apparatus and method for amplitude equalization of a signal channel such as a telephone line that does not require adjustments at the customer's premises. The equalization circuit receives a plurality of alignment signals at predetermined frequencies from the signal channel, and measures the amplitude of each alignment signal. The amplitudes are used to determine analog and digital filter transfer function coefficients. Thereafter, input signals on the signal channel are filtered such that the combined amplitude response of the signal channel and filter is substantially flat over a designated frequency range.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A circuit for connection to a signal channel for adaptively amplitude equalizing the signal channel over a predetermined channel frequency range, the circuit comprising: means for receiving a plurality of alignment signals from the signal channel, each alignment signal being at a predetermined alignment frequency;   means for measuring the amplitude of each alignment signal and for determining filter data based upon the alignment signal amplitudes;   means for storing the filter data; and   signal processor means for filtering an input signal present on the channel to produce a filtered output signal, the signal processor means including means for adjusting its amplitude response in response to the filter data stored by said means for storing, such that the combined amplitude response of the signal channel and the signal processor means is substantially flat over said channel frequency range.   
     
     
       2. The circuit of claim 1, wherein the signal processor means implement a second order filter transfer function. 
     
     
       3. The circuit of claim 1, wherein the signal processor means implement a biquadratic filter transfer function. 
     
     
       4. The circuit of claim 3, wherein the biquadratic filter transfer function includes coefficients that are determined such that the combined amplitude response of the signal channel and the signal processor means has a predetermined value at each of the plurality of alignment frequencies. 
     
     
       5. The circuit of claim 4, wherein within said channel frequency range, the biquadratic filter transfer function has an amplitude response that is substantially similar to the amplitude response of an analog filter having a transfer function of the form ##EQU10## and wherein the coefficients A and B are computed from the filter data. 
     
     
       6. The circuit of claim 5, further comprising means for receiving an analog signal from the signal channel and means for converting the analog signal into a corresponding series of digital signals, wherein the signal processor means comprise digital filtering means for receiving the series of digital signals and producing a corresponding series of filtered digital signals, and wherein the signal processor means further comprise means for converting the filtered digital signals into the filtered output signal. 
     
     
       7. The circuit of claim 1, wherein the signal processor means comprise digital filter means for producing a flat combined amplitude response over a sub-range of frequencies that at least partially overlaps said channel frequency range. 
     
     
       8. The circuit of claim 7 wherein the digital filter means implement a second order filter transfer function. 
     
     
       9. The circuit of claim 7, wherein the digital filter means implement a biquadratic filter transfer function. 
     
     
       10. The circuit of claim 9, wherein the biquadratic filter transfer function includes coefficients that are determined such that the combined amplitude response of the signal channel and the digital filter means has a predetermined value at the plurality of alignment frequencies. 
     
     
       11. The circuit of claim 10, wherein a single alignment frequency is common to two digital filter sections and their associated sub-ranges. 
     
     
       12. The circuit of claim 10, wherein in each sub-range, the filter transfer function has an amplitude response that is substantially similar to the amplitude response of an analog filter having a transfer function of the form ##EQU11## and wherein the coefficients A and B are computed from the filter data for the associated digital filter section. 
     
     
       13. The circuit of claim 12, further comprising means for receiving an analog signal from the signal channel and for converting the analog signal into a correspond series of digital signals, wherein the signal processor means comprise digital filtering means for receiving the series of digital signals and producing a corresponding series of filtered digital signals, and wherein the signal processor means further comprise means for converting the filtered digital signals into the filtered output signal. 
     
     
       14. A method for adaptively amplitude equalizing a signal channel over a predetermined channel frequency range, the method comprising the steps of: receiving a plurality of alignment signals from the signal channel, each alignment signal being at a predetermined alignment frequency;   measuring the amplitude of each alignment signal and determining filter data based upon the alignment signal amplitudes;   storing the filter data; and   filtering an input signal present on the channel to produce a filtered output signal, and compensating variations in the amplitude response of the signal channel, so that it is substantially flat over said channel frequency range.   
     
     
       15. The method of claim 14, wherein the filtering step includes the step of implementing a second order filter transfer function. 
     
     
       16. The method of claim 14, wherein the filtering step includes the step of implementing a biquadratic filter transfer function. 
     
     
       17. The method of claim 16, wherein the biquadratic filter transfer function includes coefficients that are determined such that the combined amplitude response of the signal channel and the filter means has a predetermined value at each of the plurality of alignment frequencies. 
     
     
       18. The method of claim 17, wherein within said channel frequency range, the biquadratic filter transfer function has an amplitude response that is substantially similar to the amplitude response of an analog filter having a transfer function of the form ##EQU12## and wherein the coefficients A and B are computed from the filter data. 
     
     
       19. The method of claim 18, further including the steps of receiving an analog signal from the signal channel, and converting the analog signal into a corresponding series of digital signals, wherein the filtering step includes the steps of digitally filtering the series of digital signals to produce a corresponding series of filtered digital signals, and converting the filtered digital signals into the filtered output signal. 
     
     
       20. The method of claim 4, wherein the filtering step includes the step of producing a flat combined amplitude response over a sub-range of frequencies that at least partially overlaps said channel frequency range in a signal processor. 
     
     
       21. The method of claim 20, wherein the signal processor implements a second order filter transfer function. 
     
     
       22. The method of claim 20, wherein the signal processor implements a biquadratic filter transfer function. 
     
     
       23. The method of claim 22, wherein each biquadratic filter transfer function includes coefficients that are determined such that the combined amplitude response of the signal channel and signal processor has a predetermined value at each of the plurality of alignment frequencies. 
     
     
       24. The method of claim 23, wherein a single alignment frequency is common to two sub-ranges. 
     
     
       25. The method of claim 23, wherein in each sub-range, the filter transfer function has an amplitude response that is substantially similar to the amplitude response of an analog filter having a transfer function of the form ##EQU13## and wherein the coefficients A and B are computed from the filter data for the associated sub-range. 
     
     
       26. The method of claim 25, further including the steps of receiving an analog signal from the signal channel, and converting the analog signal into a corresponding series of digital signals, wherein the filtering step includes the steps of digitally filtering the series of digital signals to produce a corresponding series of filtered digital signals, and converting the filtered digital signals into the filtered output signal.